Reactive imines: Addition of 2-aminopyrimidine to the imine bond and isolation of the aminal from the equilibrium mixture aminal/imine

Abstract

The reaction between o-vanillin and 2-aminopyrimidine leads to the formation of a single crystalline product: aminal, 1, which results from the reaction of the initially formed imine with 2-aminopyrimidine. The reaction was followed by the NMR spectroscopy. VT NMR studies prove that in solution two major species are observed: aminal and imine and their ratio depends on the time or/and temperature with the imine content increasing over time. One factor responsible for the non-typical course of the reaction between the aldehyde (o-vanillin) and 2-aminopyrimidine is the increased reactivity of the resulting imine, which easily undergoes nucleophilic addition. With the use of DFT calculations combined with experimental results we prove that the dual descriptor is the best parameter in predicting the increased reactivity of the imine bond. The calculations confirmed that the imine derived from o-vanillin and 2-aminopyrimidine is a reactive species. Another factor that determines the course of the reaction is relatively facile crystallization of aminal due to the extensive network of strong and weak hydrogen bonds. Several metal ions were tested as templating partners but no metal-imine complex could be isolated. Instead we have obtained and characterized structurally several metal ion complexes with o-vanillinate and 2-aminopyrimidine as ligands.